A duck walks into a grocery store and goes to the checkout line, looks up at the cashier and says, "Got any duck food?" The cashier says, "Um, I'm sorry, we do not. In fact, ducks aren't allowed in here, so scram."
The next day, the duck is back: "Got any duck food?" he asks the same cashier. The cashier calls in the manager. "Ducks aren't allowed in here!" the manager said. "If you ask for duck food again, I'm going to nail your little webbed feet to the floor!"
The duck leaves, but the next day, he is back again. He walks up to the manager and asks, "Got any nails?" The manager is furious. "Nails?! This is a grocery store! Of course we don't have nails!" There is a short silence. Then, confident as ever, the duck asks, "...Got any duck food?"